Japan's nuclear safety watchdog on Wednesday ordered a probe into claims the country's only working nuclear power station sits on an active tectonic fault.

The order came as Kansai Electric Power Company (KEPCO) readied to refire a second reactor at the Oi plant, western Japan, just weeks after the first unit was restarted, ending a brief nuclear-free period in earthquake-prone Japan.

A spokesman for the Nuclear and Industrial Safety Agency said KEPCO had to re-examine the Earth's crust underneath Oi, while the operator of the Shika plant in nearby Ishikawa also had to carry out further studies.

The decision came after geological experts argued both plants are likely sitting on active faults and could be vulnerable to earthquakes if tectonic plates shift.

Japan's entire stable of nuclear reactors was shut down in the months after the disaster at Fukushima when an earthquake-sparked tsunami knocked out cooling systems causing meltdowns that spread radiation over a large area.